1. Field of the Invention
The present invention relates to a transmission system and a method of correcting the tilt of the transmission system for proper tilt correction.
2. Description of the Related Art
An optical wavelength division multiplex system wavelength-multiplexes multiple optical signals into both a C band (1530-1570 nm) and an L band (1570-1610 nm) and transmits the wavelength-multiplexed optical signals. In the optical wavelength division multiplex system, when the transmission distance is long, the influence of the wavelength characteristics and the non-linear effect of optical fiber during transmission cause a difference in the optical strength levels among wavelengths. When the differences among the optical strength levels become larger, an error is caused (e.g., a reception error) in the optical wavelength division multiplex system, preventing accurate communication.
One option for suppressing the production of the difference in the optical strength level is, for example, a tilt correction method of appropriately adjusting the amplification amount of a pump LD based on a parameter set in advance in an inline amplifier. A tilt correction method is also known where a spectrum analyzer is placed at a receiving node and where tilt correction is performed by appropriately adjusting the outgoing level of each channel (ch) among transmitting nodes based on information on the difference in the optical strength level of each channel measured by the spectrum analyzer.
For example, Patent Document 1 shown below describes realizing tilt correction by detecting the optical strength level of the short wavelength side and the optical strength level of the long wavelength side of a transmitted signal light by the monitor PD (Photo Diode) of a DMUX (Demultiplexer) unit of an OADM (Optical Add Drop Multiplexer) node of the receiving side, calculating a tilt correction parameter based on the optical strength level of the short wavelength side and the optical strength level of the long wavelength side both obtained from a monitor PD, and allocating the correction parameter onto the amplifier of each relay node.
According to the disclosure of the document, when the amount of tilt of a wavelength-multiplexed signal received by an optical transmission apparatus is not in an appropriate range, the amount of tilt correction is fed back to each optical amplifier (e.g., ILA: In-Line Amplifier) located on the transmission path of the wavelength-multiplexed signal, and tilt correction is performed based on the amount of the tilt correction of each optical amplifier. This allows the optical transmission apparatus to receive a wavelength-multiplexed signal on which a proper tilt correction is performed, in other words, a wavelength-multiplexed signal having a proper OSNR (Optical Signal to Noise Ratio).
[Patent document No. 1] Japanese Patent Application Laid-open 2009-17451
The method of correcting tilt where the amplification amount of a pump LD is appropriately adjusted based on a parameter set in advance in an inline amplifier is a relatively simple correction method. However, proper tilt correction cannot be carried out based on the parameter that has been set in advance when, for example, the transmission distance considered to be the most appropriate, based on the aspect of design, is different from the actual transmission distance or when the type of fiber that is actually laid is not the one that has been expected during the design phase, and the difference in the optical strength level among channels thus cannot be overcome.
An increased number of relay stages increases the difference in the optical strength level in a cumulative manner. Thus, the use of the method of correcting tilt is substantially limited to an optical wavelength division multiplex system architecture in a metro area where the number of relay stages is low enough to prevent superimposition of the difference in optical strength levels. Placing a spectrum analyzer at a receiving node not only complicates the configuration but also results in a relatively expensive optical wavelength division multiplex system.
In a method where the amount of tilt correction is calculated based on the difference in the optical strength level between the short wavelength side and the long wavelength side and allocated to each inline amplifier, tilt correction control is performed by using a signal light. Thus, for example, when a small number of wavelength channels is used only on the short wavelength side or only on the long wavelength side, the difference in the optical strength level between the short wavelength side and the long wavelength side cannot be obtained; thus, there is concern that the tilt correction cannot be properly performed. Therefore, it is difficult to guarantee the changeover time, for example, in the case where a high-speed wavelength extension, such as route switching in mesh topology, is required.